Update CMakeLists and file locations

Move ns_file.c/h to correct location and update CMakeLists to include
ns_file.c

Author: Arto Kinnunen

connectivity/nanostack/sal-stack-nanostack/source/Service_Libs/CMakeLists.txt

@@ -101,6 +101,7 @@ target_sources(mbed-nanostack-sal_stack
         utils/ns_conf.c
         utils/ns_crc.c
         utils/ns_file_system.c
+        utils/ns_time.c
 
         whiteboard/whiteboard.c
 )